Many current systems which are based on a CDMA air interface, such as WCDMA and CDMA2000, use a linear equalizer in order to improve link and system performance and to achieve very high data rates in packet and circuit-switched connections. Applying linear equalizers to voice channels is also under consideration. Currently, rake receivers are used for voice channels. The use of a linear equalizer for both voice and high-speed data connections would significantly reduce receiver complexity and is thus highly desirable.
A typical linear equalizer is a chip-sample-level finite impulse response (“FIR”) filter, whose length is at least twice (preferably 2.5-3 times) the channel delay spread for adequate performance. The length requirement for adequate performance is particularly problematic wherever significant delay is expected. In conventional linear equalizer implementations, the greater the delay spread, the greater the linear equalizer complexity.
In the case of HSDPA, for example, 3GPP technical specifications require using an equalizer (or some other advanced receiver), which should be capable of handling a PedB channel. In addition to HSDPA or other packet switched data connections, it is possible to use a linear equalizer for voice channels (in WCDMA or CDMA2000). The performance requirements are even tighter in case of voice channels (from an equalization point of view) because the maximum delay spread to be supported can be very high. In WCDMA the requirement is 77 chips (“Case 2 channel”), which, in practice, leads to an impractical equalizer complexity.
In general, if voice channels are to be equalized, the equalizer has to be longer than the current HSPDA equalizer for robust performance in different environments. When the channel has a very long delay spread, it is probable that the channel is also sparse to a degree, i.e., the most significant channel taps are not spread evenly over the whole delay window but are concentrated inside a couple of sub-windows separated in a time domain. A conventional linear equalizer cannot utilize this sparse structure but always assumes that the channel has continuous impulse response. This can lead to unacceptable equalizer complexity.
An example of a sparse channel is the 77 chips long “Case 2” reference channel in 3GPP TS25.101 specification. Note that presented power-delay profiles are often averaged over some measurement period and thus follow some expected exponential decay curve. However, shorter averaging periods show that occasionally a large part of the energy comes through paths with very large propagation delay.
Accordingly, those skilled in the art desire methods and apparatus for performing linear equalization of channels having very long delay spreads that are significantly less complex than conventional linear equalizer implementations.
Those skilled in the art also desire methods and apparatus that capitalize on the relative simplicity of a channel having a relatively sparse impulse response-delay profile to significantly reduce the complexity of a linear equalizer used for performing equalization of the sparse channel.
In addition, those skilled in the art desire methods and apparatus for performing linear equalization more efficiently in comparison to conventional linear equalization methods that ignore signal components associated with large delays.
Further, those skilled in the art desire linear equalizers that accommodate large delay spreads often specified for voice channels and thus can be used both for high-speed data channels and for voice channels.